Brummel
2e63ed9e26
Cycle 1 of the "Cost-model graph (in R)" milestone (#148): a single ConstantCost node (flat round-trip cost per trade) plus the net-R seam — an in-graph cost_in_r stream, a net_r_equity sink (sibling of r_equity), and summarize_r folding that one stream into net_expectancy_r (replacing the scalar round_trip_cost). A run with no cost stays a byte-identical gross-R baseline. Defers the general CostNode trait, the cost-graph composite-builder, data-grounded factors, and the conviction axis. Auto-signed under /boss on the grounding-check PASS: all load-bearing existing-behaviour assumptions are ratified by green tests. Fork decisions recorded on #148. refs #148